This invention relates to photosensitive elements, particularly lithographic printing plates, which are produced by imagewise exposing a photosensitive layer on a substrate and washing away the soluble material to develop the image. More particularly, it relates to such printing plates having a metallic backing, such as aluminum, to which the light-sensitive layer is firmly bonded prior to imaging.
Diazo printing plates are widely used in the image reproduction industry, specifically as planographic and offset printing plates. Well known to the industry are the planographic printing plates produced by, for example, the Kalle division of Hoechst AG, and the ENCO group of the American Hoechst Corporation. Positive and negative-working systems are known, depending upon whether the action of light renders the photosensitive portion of the plate relatively more or less soluble than the unexposed portion. Typically, a light-sensitive diazo resin, diazonium salt, quinone diazide or photopolymerizable composition is dispersed in a binding material which is then coated onto a support surface and allowed to dry. The composition may also optionally include other constituents, such as other resins, dyes, pigments, colorants, plasticizers, light speed accelerators, photosensitizer stabilizers, fillers and the like which are well known to the skilled artisan.
Examples of suitable supports onto which the light-sensitive composition is coated are metal supports such as aluminum, steel, zinc, copper, brass, chromium, and so forth. Metal supports are distinctly preferred for long printing runs, and aluminum and aluminum alloys are usually employed. The art has found that the best results are achieved when the aluminum supports are subjected to a variety of pretreatments which enable the light-sensitive composition to be firmly bonded to the metal, in order to achieve optimum press run life and image sharpness. A currently preferred aluminum support pretreatment first anodically produces a layer of aluminum oxide on the aluminum surface, which is then reacted with a hydrophilizing polyvinyl phosphonic acid solution. This is taught in U.S. Pat. No. 4,153,461. Many other hydrophilizing agents are known to the skilled artisan.
For a thorough disclosure of methods for producing photosensitive diazo-containing printing plates and other photosensitive elements, reference is made to U.S. Pat. Nos. 3,679,419 and 3,849,392. Optically-homogeneous negative-working presensitized planographic printing plates are disclosed in U.S. Pat. No. 4,288,520.
Commercial diazo printing plates can be and are designed for use in a fairly wide range of expected press run life cycles. Examples include negative-working printing plates known as ENCO.RTM. N-25, N-50, N-100 and N-200 plates available from American Hoechst Corporation. These plates have anticipated press runs of 25,000, 50,000, 100,000 and 200,000 respectively. Some of the many factors determining the durability or press run life of a printing plate include the thickness and type of resin binder in the coating, support type and support pretreatments. It is the aspiration of the art to improve the press run life of a printing plate without diminishing the quality of the images made therefrom.